This invention relates to a method for improving corrosion resistance of bright annealed stainless steel.
Generally, stainless steel stock is first hot rolled and coiled and then, if desired, subjected to various treatments such as cold rolling, annealing, pickling, etc., and further subjected to surface finish to give surface luster or a specular surface or to form an abrasive pattern on the surface depending on the purposes of use of the product. For instance, "2D finish" is a finishing formula in which merely annealing and pickling are conducted after cold rolling. This finish gives a surface with a silver white luster and a product suitable for deep-draw working, and the worked product may be further subjected to polishing or buffing. 2D-finished stainless steel can be applied to various uses such as structural parts of aircraft, roof troughs, heat exchangers, etc. "2B finish" is a kind of luster finish which involves cold work of so slight a degree as for example, about 1-2% working, by polishing rolls after 2D finish. This finish is best suited for deep drawing of not so strict specifications. 2B-finished stainless steel is used for hospital furnishings, milk tanks, cooking utensils, tableware, building materials, and so on.
Bright annealing finish is carried out by annealing in a furnace in an inert atmosphere (usually ammonia cracked gas) after cold rolling and a succeeding cold work by polishing rolls, whereby a very beautiful surface can be obtained. Bright annealed stainless steel is used as various kinds of ornaments and articles which are not affected in their beautiful surface appearance by a slight degree of working. There exist various finishing methods for stainless steel other than those mentioned above.
As stated above, bright annealed stainless steel has beautiful surfaces and is widely used for automobile parts, bicycle parts, domestic electrical appliances, cooking utensils, construction materials, etc. However, corrosion resistance of the surface film formed by such bright annealing is unsatisfactory, especially in the case of ferritic stainless steel, and such bright anneal finished stainless steel product tends to be inferior in corrosion resistance to the product which has been abrasion finished with emery paper or other means. Therefore, in certain use environments, it is required that such bright anneal finished stainless steel be improved in its surface properties to have higher corrosion resistance.
Passivation of stainless steel by nitric acid immersion is often practiced for the purpose of improving corrosion resistance and normally by dipping in a 10-30 weight % nitric acid bath for 30 minutes-5 hours in laboratory tests. This can be applied to bright annealed stainless steel, and indeed an improvement of its corrosion resistance is provided by such nitric acid immersion. However, since the film formed by bright annealing is an oxide film of approximately 50 .ANG. in thickness formed under a high temperature, the improvement of properties cannot be achieved to a satisfactory level by simple immersion in nitric acid. Especially when it is tried to improve corrosion resistance of stainless steel bright annealed in an industrial production line, such nitric acid immersion scarecely improve the corrosion resistance because of the difficulty to secure a nitric acid immersion bath operable with so long a period of time as mentioned above in prior arts and thus limited immersion time under current practice.
The present inventors have previously developed "a method for the passivation of bright annealed stainless steel" (Japanese Patent KOKAI (Laid-Open No. 23882/84). According to this method, bright annealed stainless steel is first subjected to electrolysis and then immersed in nitric acid. This method has a noticeable effect of improving corrosion resistance of said stainless steel, but since an immersion step is required, a lengthy nitric acid tank is needed as an on-line equipment. For instance, in the combined process of electrolysis and immersion at line speed of 15 m/min and immersion time of 1 minute, the length of the nitric acid tank required for the immersion step is 15 m and that required for the electrolysis is 5 m, and thus a nitric acid tank with a total length of 20 m is required for accomplishing said process. This process requires the provision of a 4 times longer nitric acid tank than that required in the case of electrolysis alone, and the treating time is also long. Naturally, the amount of nitric acid used for filling the nitric acid tank is large, and, further, since a nitric acid solution of a higher nitric acid concentration has a higher improvement, the amount of nitric acid required is greater than that seemingly assumed, and also the amount of NO.sub.x produced during the treatment is relatively large and additional countermeasure efforts therefor are required.
In the method of the abovementioned Patent KOKAI (Laid-Open), alternating electrolysis is conducted at a current density of 5-300 mA/cm.sup.2, a liquid temperature of from normal temperature to 80.degree. C. and a wide nitric acid concentration range of 5-30 weight %. It has been considered essential to this method that the product is immersed in a nitric acid solution having a nitric acid concentration of not less than 10 weight %.
Nitric acid electrolysis is sometimes used for the purpose of descaling after annealing of products to be 2B-finished or 2D-finished, not of the product to be bright anneal finished, in the stainless steel production process, but in this case, since a higher concentration of nitric acid produces a greater descaling effect, the nitric acid concentration used is generally as high as 7-15 weight %, and further it is not intended to improve corrosion resistance.